The field of the invention is the field of lithographic masks for extremely high resolution imaging of mask features on to various substrates. In particular, the field of the invention is the field of phase shift masks, where the mask pattern is at least in part determined by the change in phase of light interacting with the surface of the mask.
Photolithography
This invention relates to the field of microlithography for the manufacture of integrated circuits, magnetic devices, and other microdevices such as micromachines. In this field the final product is manufactured in sequential manner in which various patterns are first produced in a xe2x80x9cresistxe2x80x9d material with each pattern subsequently defining a product attribute. The xe2x80x9cresistxe2x80x9d materials, generally polymer compositions, are sensitive to light or other forms of radiation. The patterns are formed in the resist by exposing different regions of the resist material to different radiation doses. In the bright (high dose) regions, chemical changes take place in the resist that cause it to dissolve in a chemical bath or be etched away by a gas or plasma more easily (for positive resists) or less easily (negative resists) than in dim (low dose) regions. If the radiation flux on to the resist is too little, the resist is said to be underexposed, and in general the underexposed area of the positive resist will not all dissolve away, and the negative resist will only partially dissolve away when the resist is xe2x80x9cdevelopedxe2x80x9d, (IE dissolved or etched). With sufficient radiation flux for exposure, the unexposed and exposed areas of the resist will be developed with one area all dissolved away and the other area remaining as a protective coat for the next step in the wafer processing. For a radiation flux that is too great, the resist is overexposed and the areas exposed tend to xe2x80x9cbloomxe2x80x9d out and etched out lines, for example, are wider than they would be with just a sufficient exposure. For a good resist, a xe2x80x9cmanufacturing windowxe2x80x9d exists between such underexposure and overexposure fluxes.
Patterned Resist and Process
The bright and dim regions are formed using an exposure tool which generally transfers corresponding features to the resist from a mask or reticle. The masks or reticles are formed from mask substrates, which are plates of quartz or other material transparent to the radiation used for exposing the resist, coated with an opaque material such as chrome. The chrome is etched away in a pattern to form the mask. The radiation employed may be (but is not limited to) ultraviolet light and x-rays, and the regions of the mask that are opaque and transparent form a pattern of bright and dark when illuminated uniformly. In the most common implementation of this technology, a projection lens forms an image of the mask pattern in the resist film on a planar substrate. That image comprises the high and low dose regions that produce the resist pattern. When some form of light is employed in this process, it is called photolithography.
Wavefront Engineering
The patterns formed in the resist are not identical to those on the mask, and the methods of obtaining the pattern desired for the ultimate manufactured device in spite of deficiencies in the microlithography process is called xe2x80x9cwavefront engineering.xe2x80x9d Among the various devices used for this purpose are phase shifting masks (PSM)sxe2x80x94which create desired dark regions through interference. Phase shift masks were first published by the inventor of the present invention in a paper entitled xe2x80x9cImproving resolution in photolithography with a phase shifting mask,xe2x80x9d M. D. Levenson, N. S. Viswanathan, and R. A. Simpson, IEEE Trans. Electron Devices ED-29, 1828-1836 (1982). Since that time, there have been hundreds of patents and thousands of papers issued containing the phrase xe2x80x9cphase shift maskxe2x80x9d. Phase shift masks allow production of features with smaller linewidths than conventional photolithography, in that the low dose portions of the photoresist are much narrower than the high dose portions. The pitch, however, of such narrow linewidth features is limited to xcex/2 NA., where xcex is the wavelength of the light used to expose the resist and N.A. is the numerical aperture of the optical system used to expose the resist.
There are presently two types of PSMs in use: weak-PSMs such as the Attenuated-PSM and strong-PSMs such as the Alternating-Aperture-PSM. These two differ in that the weak-PSMs have only one type of bright feature, while the strong-PSMs contain two types of bright features identical except for the optical phase, which differs by xcx9c180xc2x0. See, for example, M. Shibuya, Japanese Patent Showa 62-50811, M. D. Levenson et. al. IEEE Trans. Elect. Dev. ED-29, 1828-1836 (1982), and M. D. Levenson, Microlithograpy World 6-12 (March/April 1992).
Typically, narrow and xe2x80x9cdarkxe2x80x9d lines may be produced by phase shift methods, where the phase shift arises from two neighboring areas of the mask which shift the phase of light interacting with the mask by 180xc2x0. The two areas of the mask are separated by a border which is a straight line on the mask, and when the mask is imaged on a resist, a very narrow line with little or no illumination between two bright areas results. Such narrow lines are valuable, for example, as gate lines in a semiconductor device. However, the length of the lines is much larger than the width of the line.
There is a demand for features which are holes in the resist with as small an area or as small a diameter as possible. Such holes are used, for example, to produce contacts to underlying conducting lines or other parts of semiconductor devices. FIG. 1 shows a sketch of the intensity of the light produced by three prior art methods for producing such holes. A T-mask is just a hole in the chrome covering of a normal mask substrate which produces the intensity given by curve 10. The minimum diameter of the light pattern is determined by diffraction, and does not get smaller as the hole in the chrome is reduced beyond a certain point. The Attenuating PSM (curve 14) and Rimshift PSM (curve 12) technologies result in somewhat smaller diameter bright areas.
U.S. Pat. No. 5,807,649 teaches a double exposure system for exposing a photoresist using a phase shift mask and with a second mask to expose unwanted dark areas left by the phase shift mask. U.S. Pat. No. 5,620,816 teaches a double exposure system where a chromeless phase-edge shift mask is used to expose all of the photoresist except on lines running in rows and/or columns, and then a customized mask is used to expose unwanted portions of the lines and/or columns. If the same or another chromeless phase-edge mask is used to expose the same resist wherein dark lines run perpendicular to those dark lines left by the first exposure, some of the unexposed areas of the resist left by the first exposure would be exposed, and an array of unexposed spots with very small diameter will result. The prior art does not show any method of producing very small regions of unexposed photoresist using a single exposure.
PSM Design
Various Electronic Design Automation (EDA) tools are known for preparing the patterns used in conventional and phase-shifting masks. In addition, OPC tools alter those patterns to account for the realities of the exposure systems. It is also known that the pattern of apertures on the phase-shifting mask need not correspond closely to the ultimate circuit pattern, at least not when a conventional block-out mask is employed for a second exposure on the resist film in concert with a first exposure made using an alternating-aperture PSM. Such second exposures erase anomalies due to phase-conflicts. Numerical Technologies, Inc., in U.S. Pat. No. 5,858,580, in particular, has demonstrated the In-Phase design system which employs a block-out mask similar in geometry to the ultimate circuit feature along with an alternating-aperture PSM composed of pairs of small apertures (shifters), one of which has 0xc2x0 phase, while the other has 180xc2x0xe2x80x94which define the narrowest dark features between them.
Related U.S. patents issued to the inventor of the present invention are U.S. Pat. Nos. 6,287,732 and 6,251,549 issued Sep. 11, 2001 and Jun. 26, 2001 respectively. Related U.S. patent applications by the inventor of the present invention are applications entitled xe2x80x9cGeneric phase shift maskxe2x80x9d filed Sep. 6, 2001 as U.S. patent application Ser. No. 09/947,336 now U.S. Pat. No. 6,479,196 and xe2x80x9cPhotolithography method and apparatusxe2x80x9d filed Feb. 25, 2002 as U.S. patent application Ser. No. 10/083049. The above identified references, U.S. patents and U.S. patent applications are hereby incorporated by reference.
It is an object of the invention to provide apparatus, methods, and systems for the production of very small area features on a device.
It is an object of the invention to provide a phase shift mask substrate having features which provide for production of very small area features on a device.
It is an object of the invention to provide a phase shift mask substrate having features which provide for production of very small area features on a device.
It is an object of the invention to provide a phase shift mask having features which provide for production of very small area features on a device.
It is an object of the invention to provide a method of making a phase shift mask substrate having features which provide for production of very small area features on a device.
It is an object of the invention to provide a method of using a generic phase shift mask substrate having features which provide for production of very small area features on a device.
It is an object of the invention to provide a method of using a generic phase shift mask substrate having features which provide for production of very small area features on a device.
It is an object of the invention to provide a method for making photolithographic exposures of two dimensional arrays of small area features having pitch  less than xcex/NA, where xcex is the wavelength of light used by the photolithographic exposure system and NA is the numerical aperture of the photolithographic system.
The present invention is a system, apparatus and method to produce very small areas features on a device. A phase shift mask substrate is produced where an area on the mask substrate has a phase shift variation around a point, such that light interacting with the area around the point has the phase of the light shifted so that there is a sharp and deep minimum of the light intensity in a first area corresponding to the point, and an annular second area surrounding the first area where the light intensity is much greater than the minimum through the entire annular area. The phase shift varies in a spiral pattern about the point so that the phase shifts suddenly by approximately 180xc2x0 when measured along lines crossing the point, and the phase shifts slowly or suddenly by an amount less than 130xc2x0 or greater than 230xc2x0 when measured along any other line traversing an area on the mask corresponding to the second annular area. In particular, the most preferred embodiment has a substrate with a one turn spiral staircase like a set of xe2x80x9cstepsxe2x80x9d etched down from the surface into the substrate, with the height of each step producing a phase shift from step to step of less than or much less than 130xc2x0, and the phase shift from the bottom step back up to the first step of greater than approximately 230xc2x0.